I found the movie entertaining on a superficial level, but it is no great work of art like The Godfather or Goodfellas.
As Turnball stated, the film bares little resemblence with reality(other than the fact that there was a bootlegger called Al Capone, and Elliot Ness got him sent to jail for tax evasion).
But my biggest problem with the film was De Palma. I have never rated him as a director. Although he is capable of creating suspense, he also has a talent for giving films the wrong mood and atmosphear. His films often lack subtlety and he often allows events to become far too contrived. For example the scene where Ness falls off the roof, but (as luck would have it) there is a peice of scafolding sticking out at the exact place where he falls, is just too rediculous. Then there was the shoot-out at the Odessa steps. Did I say "the Odessa steps, I meant the train station. That scene was a highly exilerating peice of action, but Ness and Garcia being able to shoot all of Capone's men AND save the baby is just to improbable (in Eisenstein's Battleship Potymkin, the baby gets killed, which is what would enevitably happen in real life).
I also didn't think Connery was any good atall, but I guess that's just a matter of view point.
I think that had the film been a straight falward action film and not an attempt to re-write history, it would have been a lot better. As it stands, it is a sensationalistic peice of Hollywood film making.
However, regardless of this, I must admit that the film is very entertaining. As for DePalma, I think Carrie was the only film he really did a good job on. Did I mention that DeNiro's performance in The Untouchables is the best thing in the film.